In some applications it is desirable to limit the output voltage of power supplies when a high impedance, such as an open circuit load is presented to the output of the power supply. This is the case for power supplies which provide power to subscriber lines of a communication system. While it is desirable to provide a nearly constant power output to subscriber lines in order to accommodate lines of varying length and line impedance, it is also desirable to avoid significantly higher than normal output voltage levels. A number of different power supply designs employ a transformer with a switching circuit controlling input current to the primary of the transformer on a pulsed basis. One particular design uses a feedback winding coupled to the primary winding of the transformer to control the current flow in the primary. Such prior art power supplies, however, have the disadvantage that due to imperfection in the circuit elements the output voltage tends to rise significantly when the open circuit load condition is encountered. Furthermore, output voltages of such prior art power supplies tend to vary with variations in input voltage. Another problem in the design of power supplies for subscriber lines is the ability to detect accurately the on-hook and off-hook states of a connected subscriber set.